In order to SEND email reliably, a mailserver MUST meet the following DNS requirement:

In order to SEND email reliably, a mailserver MUST meet the following DNS requirement:

• SPF and DKIM records
• A valid MX record
• A PTR record which matches the email domain
• A valid PTR record 

 

EXPLANATION

A sending mailserver shall have a valid PTR (reverse DNS lookup) record for its public IP address. That is, the PTR record must return the same IP address if submitted to a DNS lookup. If this is not the case, the recipient is likely to reject the mail as possible spam. (If IPv6 is used this also applies here, a point often overlooked)

MX records determine which computers accept inbound mail for

SOURCE

https://www.studiocoast.com.au/knowledgebase/194/email/support.aspx